Superconductors are materials which carry electrical current without dissipation. However, feeding electrical current into a superconductor generates heat dissipation in the contacts and degrades maximum attainable current value. This degradation can be minimized by creating electrical contacts with low resistance.
The superconducting state is destroyed if the parameters of the experiment exceed some critical values. These are critical temperature Tc and critical magnetic fields, the higher of which called the upper critical field, Hc2 destroys superconducting state completely. The maximum density of current which the superconductor can carry without dissipation is called critical current density Jc. This value, in addition to temperature and magnetic field, depends on both intrinsic properties of materials used and the form of their preparation.
There is an ongoing effort to find superconductors with high intrinsic critical parameters. Discovery of superconductors with high transition temperatures, well exceeding the boiling point of liquid hydrogen, 20 K, opens the way for using high temperature superconductors for a variety of application. Just a few superconductors are known with transition temperatures on the order of 40 K, offering prospective for technological applications at 20 K. These include numerous compounds based on copper oxides and generally referred to as superconducting cuprates (Tc up to 135 K at ambient pressure) and MgB2 (Tc=39K). A new family of materials based on iron-arsenide compounds known as iron pnictides, which are generally represented by AFe2As2 where A can be Ca, Sr, Ba or combinations thereof, offer a number of superior material parameters as compared to the cuprates. These include low anisotropy of the upper critical fields, high values of the upper critical field and small anisotropy of the superconducting critical currents.
Superconductivity can be induced in the iron pnictides by partial chemical (elemental) substitution (doping) of either of the three elemental constituents (A, Fe, As) and/or by application of pressure. Substituted iron pnictides can be represented by (A1-xD1x)(Fe1-yD2y)2(As1-zD3z)2, where A stands for alkali earth elements Ca, Sr, Ba, Eu or their combinations, and various elements can be added as dopants D1, D2 and D3.
Superconducting materials based on K-, P-, Co-, or Ni-doped BaFe2As2, with high transition temperatures to the superconducting state (above 20 K) were discovered in mid-2008 and their electrical properties were tested using commonly accepted in-laboratory electrical contact making techniques that involved attaching electrical conductor wires (e.g. silver wires) to the superconductor using silver epoxies or silver paint. However, these electrical connections were characterized by high contact resistance on the order of 10−3 ohm-cm2, and are not suitable for high current applications.
Feeding of electrical current into the superconductor generates heat dissipation in the contacts and degrades maximum attainable current value. This degradation can be minimized by creating contacts with low resistance. However, the selection of contact materials heavily depends on the surface reactivity of the materials in contact. For unknown material, determination of right combination is impossible without heavy experimenting. For example, Indalloy Corporation provides a research soldering kit, targeting soldering non-standard metals. The number of alloys in the kit is above 280, which go in combination with 5 different fluxes. This gives more than 1400 combinations to test. Even when a mechanically strong soldered joint is made, there is absolutely no guarantee that it would give suitable contact resistance. The same is true with different approaches, used for contact making. These include, but are not limited to, different variants of vacuum evaporation (thermal, plasma, reactive magnetron sputtering, laser ablation). During contact making, new binary and ternary compounds are easily formed at the interface, which adds to the poor predictability of the contact electrical performance.